They fix this by opening a new window and having you login directly to Disqus.com so the cookie can be set. Disqus login still works with third-party cookies disabled...

However, don't use Disqus because they inject vulnerable JavaScript onto the page from unvetted ad networks and inject their affiliate links into every external link on your page[1].

[1]: https://en.wikipedia.org/wiki/Disqus#Affiliate_links_and_thi...